Calculate the size of kernel text correctly if it spans multiple prom mapping entries.  This is necessary on the v1280 where the firmware mixes 4MB and 8KB mappings to map kernel text.
Calculate the size of kernel text correctly if it spans multiple prom mapping
entries.  This is necessary on the v1280 where the firmware mixes 4MB and 8KB
mappings to map kernel text.

tested by miod@ and nick@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars